1. Field of Invention
The present invention relates to processing radio signals in a CDMA system, and more particularly to signaling allocation information including, for example, which codes in which timeslots have been assigned to mobile terminals.
2. Description of Related Art
A cellular radio system or network may include multiple base stations and a number of mobile terminals. A base station may also be referred to as a node B. A mobile terminal may also be referred to as a mobile, a mobile radio, mobile transceiver or user equipment (UE). A mobile terminal may be fixed, stationary, portable, moveable and/or moving within a cell or between cells. A single base station may serve multiple mobile terminals by transmitting separable transmissions to each mobile terminal. A mobile terminal may determine which signals were directed to it and separate those signals from signals directed to other mobile terminals.
Signals may be separated in one or more domains. For example, signals may be separated in the time domain by transmitting time division multiple access (TDMA) modulated signals. Additionally, signals may be separated in the frequency domain by transmitting frequency division multiple access (FDMA) modulated signals. Also, signals may be separated in the code domain by transmitting code division multiple access (CDMA) modulated signals. Signals may be separated in the spatial domain by transmitting signals from collocated antennas. A cellular radio system may also employ a combination of these and/or other separation techniques.
In a CDMA system, multiple users may be supported via spread spectrum techniques. In a direct sequence CDMA system, a data payload is encoded with a code that may be orthogonal or pseudo-orthogonal to other codes. A mobile terminal may receive a CDMA modulated signal and may perform various demodulation operations such as matched filtering with one or more codes assigned to that mobile terminal.
When a base station modulates and transmits a CDMA signal encoded with a particular code, a mobile terminal may use a matched filter and the particular code assigned to it to produce a high output out of the matched filter. A matched filter using the particular code will produce a low output for signals directed to other mobile terminals, which are assigned other codes. As a result, a mobile terminal decodes only those signals with high matched filter outputs and therefore directed to it. Similarly, a mobile terminal rejects those signals with a low matched filter output and presumably directed towards a different mobile terminal.
Direct sequence CDMA systems commonly use either a frequency division duplex (FDD) scheme or a time division duplex (TDD) scheme. In an FDD system, communication between a mobile terminal and a base station occurs on two non-overlapping frequency bands. In a TDD system, communication between a mobile terminal and a base station may occur within a single frequency range. In either case, a data payload is transmitted between a mobile terminal and a base station. Uplink data or uplink traffic is transmitted from a mobile terminal to a base station. Downlink data or downlink traffic is transmitted from a base station to a mobile terminal.
In an FDD system, frequency separation is employed. Uplink traffic is transmitted at one center frequency and downlink traffic is transmitted at a different center frequency. The uplink and downlink may operate concurrently. That is, a mobile terminal may transmit data to a base station on an uplink at the same time that the base station is transmitting data on a downlink to the mobile terminal. The frequency separation in FDD systems ensures that the uplink does not interfere with the downlink.
In contrast, a TDD system employs temporal separation. A TDD system may transmit uplink and downlink data within a single frequency range, but at different times. An air interface link between a group of mobile terminals and a base station in a TDD cell may be organized in the time domain as a sequence of frames. Each frame may be arranged as a set of timeslots. Some timeslots may be allocated to uplink traffic while other timeslots may be allocated to downlink traffic. Each timeslot may be further subdivided in the code domain using a set of codes. Data is separated into codes with different orthogonal or pseudo-orthogonal codes from the set of codes. In order to facilitate decoding, the data transmitted on the code is separated into a data payload encoded with different orthogonal or pseudo-orthogonal codes, a training sequence and a guard period; the resulting structure consisting of data payload, training sequence and guard period is referred to as a burst.
To implement spatial diversity, a TDD base station may use two or more antennas: During downlink timeslots, a first set of bursts transmitted during a timeslot through a first antenna may be directed towards first group of mobile terminals and a second set of bursts transmitted during the same timeslot through a second antenna may be directed towards second group of mobile terminals. The first and second groups of mobile terminals may contain the same and/or different mobile terminals.
A base station may allocate a group one or more codes from one or more timeslots of a frame for downlink traffic. This allocation may be made for a first mobile terminal. A mobile terminal may receive data at higher rates with each additional code of a timeslot assigned to it. Furthermore, a base station may make these allocations in concurrent timeslots, each timeslot to be transmitted simultaneously over a different antenna. The base station informs each mobile terminal that it will receive downlink data by informing the mobile terminal of its allocated timeslots and codes. A mobile terminal then monitors the timeslots and decodes signals with the codes allocated to that mobile terminal.
FIG. 1 illustrates a typical frame structure for a TDD cellular radio network. A single TDD radio frame 100 may consist of 15 timeslots (Timeslots 1-16). Each timeslot consists of a set of bursts, the set may have up to 16 active coded signals using Codes 1-16. A base station transmits (on the downlink) zero, one or more bursts with one or more coded signals contained in each burst. Similarly, one or more mobile terminals each transmit zero, one or more bursts on the uplink, each burst containing one or more coded signals. The separate bursts on the uplink may be received as a single combined set of bursts by the base station.
A network may split a frame into downlink timeslots 101 and uplink timeslots 102. A network may make a symmetric division of downlink and uplink timeslots when mobile terminals transmit a similar volume of data as they receive. A network may configure an asymmetric service when a majority of the data flows in one direction. For example, internet traffic typically occupies a much larger volume of downlink data than up uplink data.
Frame 100 is configured to have 10 downlink timeslots (Timeslots 1-10) 101 and 5 uplink timeslots (Timeslots 11-15) 102. Also shown is allocation information for three mobile terminals (Terminals 1-3). The network has allocated four codes (Codes 3-6) of a single timeslot (Timeslot 3) to Terminal 1. These four codes are not shared with other mobile terminals. Also, the timeslot happens not to be shared with other mobile terminals and no codes are used in the timeslots just before or just after, therefore Terminal 1 should not suffer from intracell interference.
The network has allocated 6 codes to Terminals 2, namely Codes 2 & 3 in each of Timeslots 5-7. The network has also allocated 8 codes to Terminals 3, namely Codes 6 & 7 in each of Timeslots 5-8. Signals transmitted to Terminals 2 and 3 are multiplexed in each Timeslots 5, 6 and 7, therefore, the signals in these timeslots directed to one mobile terminal may interfere with signals directed to the other mobile terminal. Timeslot 8 is not code multiplexed with any other terminal except Terminal 3, therefore, Terminal 3 does not receive interference from other codes in Timeslot 8.
A typical TDD timeslot burst may contain multiple coded signals. Each burst may be considered as including three portions: a data payload, a training sequence and a guard period. Although the order and size of these portions within a burst may vary from system to system, a training sequence will typically be inserted as a midamble between two halves of the data payload. Alternatively, a training sequence may be placed at the head (preamble) or tail (postamble) of the data payload. Additionally, the guard period will typically be appended to the end and/or the beginning of the data payload and training sequence.
FIG. 2 illustrates segments of a TDD coded signal 200 from a single burst of one timeslot. The coded signal 200 includes a data payload (part 1) 201 followed by a midamble training sequence 202 followed by a remainder of the data payload (part 2) 203 followed by a guard period 204. This format of data payload 201, 203, training sequence 202 and guard period 204 may be used in cellular radio networks such as in a UTRA TDD mode system as specified by the third generation partnership project (3GPP).
In each timeslot, a set of bursts may be transmitted, where the burst contains one coded signal for each active code. Each coded signal may contain a unique training sequence or may contain a training sequence used by one or more of the other coded signals. A set of bursts may be distorted by a propagation environment in which a cellular radio system operates. The environment may provide multiple paths between a base station antenna and a mobile terminal antenna. A resulting radio channel may not be a perfect channel but rather a channel that combines delayed versions of a transmitted signal. For example, a signal transmitted from a base station and directed towards a mobile terminal may take multiple paths and these signal paths may be of different lengths. Hence, a burst or a signal may arrive at the mobile terminal as multiple facsimiles of the transmitted signal and each facsimile may arrive at different times due to the different length paths. A sequence of symbols within the signal may thus destructively interfere with each other.
For example, a transmitted signal traveling a short path arrives at a receiver first. The same transmitted signal traveling a longer path may appear at the receiver as a delayed version of the first received signal. Therefore, a first symbol traveling a longer path may arrive at a receiver at the same time a subsequent symbol traveling a shorter path arrives at the receiver. The mobile terminal may receive a signal comprised of a combination of one or more delayed versions of the transmitted signal. This phenomenon of overlapping symbols is known as intersymbol interference and may be caused by multipath propagation.
Intersymbol interference caused by multipath propagation also reduces orthogonality among signals having different codes. This loss of orthogonality among codes leads to a degradation in correlation properties and lower overall system performance. Furthermore, intersymbol interference may increase interference experienced by two signals having different codes transmitted in the same timeslot.
Referring to FIG. 1 for example, intersymbol interference may cause a loss of orthogonality among Codes 2, 3, 6 and 7 directed to Terminals 2 and 3 in each of Timeslots 5, 6 and 7. Additionally, intersymbol interference may cause a loss of orthogonality between Codes 6 and 7 of Terminal 3 in Timeslot 8. Unless a network employs mitigation techniques to reduce the impact of multipath, system performance may degrade.
A mobile terminal receiver may receive a signal containing traffic directed both to itself and to other mobile terminals. The mobile terminal receiver uses its assigned codes to extract data directed just to it. The encoded data directed to other mobile terminals in the same timeslot and from the same or a different antenna may interfere with the mobile terminal's reception and data extraction. A base station may increase its transmit power to compensate and overcome a perceived interference. Increasing transmit power, however, also increases interference in a network. Therefore, other means to process interfering signals may be useful.